Coating apparatus employing an air knife doctor



' Filed Nov. 4, 1959 COATING APPARATUS EMPLOYING AN AIR KNIFE DOCTOR 2 Sheets-Sheet 1 INVENTOR WELCOME WILLARD WAMSLEY BY na o A GENT June12, 1962 w.w.wAMsLEY 3,038,441

' TUS EMPL K INVENTOR WELCOME WILLARD WAMSLEY AGENT United States Patent M 3,038,441 COATING APPARATUS Eh IPLOYING AN AIR KNIFE DOCTOR Welcome Willard Warnsley, Metuchen, N.J., assignor to E. I. du Pont de Nemours and (Iornpany, Wiimington, Del., a corporation of Delaware Filed Nov. 4, 195?, Ser. No. 85%,860 4 Claims. (Q1. 118-63) This invention relates to coating web materials and more particularly to coating web materials with a liquid layer using a coating apparatus equipped with an air knife doctor. Further, the invention relates to an apparatus for improving the uniformity of layers coated with air knife coating devices. Still more particularly the invention relates to coating equipment comprising an air knife doctor and auxiliary equipment to improve the smoothness of the layer at coating speeds in excess of 100 feet per minute.

The use of air knife doctors or air jets impinging on the coated surface to remove the excess coating and to smooth the remainder of the coating material to produce a uniform layer is well-known. This method of smoothing a coating is, of course, far superior to older methods using mechanical scraper blades or doctor rolls for this purpose. However, as the coating art has progressed and higher coating speeds have been sought it has been necessary to modify the design of existing air knife doctors to overcome turbulence which, in turn, causes uneven coatings. Air knives have been designed so that a minimum of turbulence is encountered in the plenum chambers and the throats of the air knives in order to produce a uniform air stream that flows in a straight line without tending to diverge or converge. Also, by means of design modifications the air stream has been freed from pressure or density variations along its length from the orifice outwards. Another aspect of design improvement shown by the prior art has been to reduce the vibratory motion of the orifice lips caused by the air stream which in turn causes uneven layers. One further improvement has been directed to decreasing turbulence by controlling the stream of air after leaving the air knife lips. This improvement resides in either attaching to, or forming integrally with, the lower lip of the air knife a recess and a curved or streamlined surface in the path of the air stream passing between the web and the leading lip of the air knife. A variation of this takes the form of a rotating curved surface positioned similarly to the curved surface described above or positioned in a socket and forming the terminal part of the leading or lower lip of the air knife in such a manner that the inner throat surface of the lower lip is tangent to the curved rotating member.

While the above design modifications have brought about improvements in the coating art, many difiiculties have become evident as higher viscosities and faster coating speeds have been attempted. During normal air knife coating, liquid in excess of the final desired coating weight is picked up by a moving web as it passes in contact with the surface of the liquid to be coated. The excess material is blown off by an impinging air stream. The uniformity or degree of smoothness with which the coatings can be applied by the air knife technique depends, in addition to the design of the air knife, on the viscosity of the material to be coated, final coating thickness and the combination of coating speed and air knife pressure used. As the coating speed is increased, certain defects become noticeable. Severe streaks are usually the first defect encountered and these are caused by the excess liquid being 3,38,44l Patented June 12, 19%2 blown back by the air knife in the form of heavy rivulets. Generally, this condition occurs in the range of 100 to 110 feet per minute and a smooth and uniform coating cannot be obtained on an impervious web at greater speeds Without mechanical smoothing or smoothing by material runout after air knifing. Even at speeds less than 100- 110 feet per minute coating quality is dependent on the smoothness of liquid flow below the air knife. A combination of high viscosity, low coating weight and high air knife pressure can cause defects in the final coating at speeds below 100-110 feet per minute because of excessive turbulence in the flowing liquid below the air knife.

An object of this invention is to provide an apparatus for coating web materials with a liquid coating material. Another object is to provide an apparatus for uniformly applying a liquid coating material to a web moving at high speeds. A further object is to provide an apparatus and method for uniformly coating films with photographic emulsions and auxiliary layers at speeds greater than 100 feet per minute. A still further object is to provide an apparatus for coating continuous webs utilizing an air knife doctor that eliminates streaks and uneven coatings caused by turbulence and rivulets during high speed coating operations. Other objects will appear hereinafter.

These and other objects are accomplished by the following invention which comprises an apparatus and method for applying a smooth coating of desired wet thick ness onto a moving web including means for applying to the web a layer of coating material of at least twice the desired wet thickness, an air knife doctor means for directing a stream of air at said layer at an impingement line transverse to the direction of web travel, the improvement which consists in placing a taut wire or rod parallel to said impingement line at a distance p from the coated web, at a distance q below the impingement line and said wire or rod having a diameter d wherein:

d is a distance equal to from 4 to 20 times the desired wet thickness,

p is a distance equal to from 1 to 6 times the desired wet thickness, and

q is a distance equal to from 1 to 5 times d.

This invention will now be specifically described with reference to the accompanying drawing wherein:

FIG. 1 is an end sectional view of a preferred embodiment of the invention.

FIG. 2 is a large scale view of a portion of the apparatus showing the critical distances d, p, and q.

FIG. 3 is a perspective view of the apparatus shown in FIG. 1 to better illustrate the relationship of various parts of the invention.

FIG. 4 is a large scale view of a portion of an embodiment of the apparatus.

Referring now to FIG. 1 which illustrates a preferred coating apparatus comprising a web 1 which travels around a coating roll 2. The portion of the web at the bottom of the coating roll skims the surface of the coating liquid 3 which is contained in the pan 4. The pan has a weir 5 and an overflow trough 6. An air knife doctor having an upper blade 7 and lower blade 8 is situated adjacent to the roll. The air knife doctor can be one such as described in Terry, US. Patent 2,139,628 and the location of blades 7 and 8 can be reversed if desired. In conjunction with the air knife doctor, a taut wire or rod 9 is placed parallel to the coating roll just below the point of impingement of air from the air knife onto the web. The positioning of wire or rod 9 is critical to the invention and will be discussed at length in conjunction with FIG. 2.

The web 1 is contacted with the coating liquid 3 as it travels around the roll 2. The coating liquid 3 to be coated is fed to the coating pan 4 by any suitable means in such a manner that there is a slight continuous and uniform overflowing of the liquid over the weir 5 of the coating pan into the overflow trough 6. By means of the forces of viscous drag, an amount of liquid in excess of that desired on the final coated web is picked up by the web moving over the coating roll and moved toward the air knife doctor. The excess liquid is forced back to the coating pan assembly by the doctoring action of the air stream from the air knife. The excess liquid below wire or rod 9 tends to be turbulent and in the form of heavy rivulets especially at coating speeds of 100 feet per minute and higher depending on the viscosity of liquid, coating weight desired and air knife pressure. However, due to the presence of the taut wire 9 heavy rivulets and areas of turbulent flow form only below the wire 9 and the flow of liquid between the wire 9 and the point of air impingement on the web is left smooth and devoid of visible turbulence.

The wire 9 acts as a focal point for the formation of a region of smooth liquid flow out of which the final coated web 10 is drawn. Rivulets and turbulent liquid flow directly below the point of air impingement are eliminated. The uniformity of the final coating is improved, and coating speeds beyond the normal limit of 100 to 110 feet per minute can be obtained without sacrificing coating quality.

Referring now to FIG. 2 which shows in large scale view a portion of the apparatus without the presence of coating liquid, the critical location of wire 9 will be discussed. To achieve the improved coatings previously described, the diameter d of the wire 9, the distance q of the wire below the point of air impingement 11 on the web 10 and the clearance p of the wire from the web are important. The distance q of the wire below the point of impingement 11 should be kept as small as possible While at the same time maintaining a continuous pool of liquid along the top edge of the wire. The distance q can be from 1 to 5 times diameter d. A value of 0.10 inch is generally a suitable value for q when coating photographic layers; however, values from 0.06 inch to about 0.25 inch may be used depending on the specific fluid properties and coating speeds. Where q is greater than 0.25 inch, rivulets and turbulent areas may form above the wire and the value of the novel arrangement may be lost. The distance p of the wire from the web is preferably held within a range of about 1 to 6 times the wet thickness of the final desired coating. If the distance p is too great it will allow air from the air knife doctor to blow behind the wire and destroy its smoothing effect. If the distance p is too small, uneven mechanical doctoring by the wire will take place and final air knife smoothing will not take place. The diameter of the wire d should be as large as permissible while still maintaining complete liquid coverage along its length; a diameter from about 4 to times the final desired wet thickness is suitable. If the wire is too small in diameter, no smoothing effect will be obtained. When coating photographic emulsions, diameters from about 0.02 inch to about 0.1 inch are preferred.

FIG. 3 is a perspective view of the apparatus shown in FIG. 1 which better illustrates the relationship of the various parts of the invention; like reference numbers are used to refer to corresponding elements previously described.

FIG. 4 shows in large scale view a portion of an embodiment of the apparatus wherein two similar, thin, rounded longitudinal obstructions 9 and 9' are utilized. Obstruction 0' is located below obstruction 9 equidistant from the web. Its diameter and location below the point of impingement 11 fall within the ranges as set forth above for d and q, respectively.

The other variables in the arrangement of the apparatus are those encountered in normal air knife coating. The effects of changing these variables within their normal working ranges are well-known to those skilled in the art and such variables are independent of the function of the wire in obtaining smoother coatings at increased coating speeds.

The invention will now be illustrated in and by the following examples which illustrate the application of the invention to coating molten photographic gelatinosilver halide emulsions and auxiliary layers on suitable impervious film supports. The air knife variables which are not critical to the invention are as follows for all examples:

Coating roll diameter=3 /s inches Air knife lip openings=0.035 inch Air knife clearance from web=0.l35 inch Angle of air impingement, 0=l0 (shown in FIG. 1 as formed by the extension of the lower air doctor knife blade 8 and a radius of the coating roll drawn to point of impingement) Web to Weir clearance=0.10 inch (approx) Example I A gelatin silver iodobromide emulsion containing 13% solids of which silver halides represented 8% of the total solids was made in a conventional manner. The emulsion had a viscosity of approximately 5 centipoises. The coating apparatus was of the type shown in the drawing and included a wire diameter d of 0.063 inch, said wire being placed a distance q of 0.12 inch below the point of air impingement and a distance p of 0.006 inch from the web. The emulsion was introduced into the coating pan assembly as described above and coated at 150 feet per minute with an air knife pressure of 0.65 inch of water. A visibly smooth and uniform coating was obtained having a weight of 97 mg./dm. of silver halide expressed as silver bromide which corresponds to a wet coating thickness of 0.004 inch. A control operation under identical conditions but in the absence of the taut Wire below the point of impingement of air from the air knife yielded a coating with severe streaks running parallel to the direction of travel of the web.

Example 11 A non-halation layer for photographic elements was made using a 7% aqueous gelatin solution containing a suitable dye and having a viscosity of approximately 10 centipoises. A coating apparatus of the type shown in the drawing was used in which the taut wire diameter d was 0.037 inch and it was placed at a distance q of 0.1 inch below the point of impingement of air from the air knife and at a distance p of 0.02 inch from the web. The non-halation gelatin solution was coated at feet per minute with an air knife pressure of 1.08 inches of water. A visibly smooth coating was obtained having a coating weight of approximately 75 mg./dm. of gelatin which corresponds to a final wet thickness of 0.0035 inch. A similar operation in the absence of the taut wire yielded a coating with severe streaks in the direction of travel of the web.

The novel feature of using a taut wire below the point of impingement of air from the air knife doctor onto the coated web is not limited to wires per se, but may include metal or plastic rods and the like. More than one wire may be used arranged one below the other and following the periphery of the coating roll. Further, wires or rods other than those having circular cross-sections may also be used. For example, those having a tear-drop crosssection or other hydrodynamically favorable shapes may be used. The wires or rods can be alfixed to the apparatus by suitable means known in the art so as to be adjustable as may be desired. The invention is not limited to coating a web on one side only. In place of a coating roll as shown in the drawing, the invention may be used advantageously where it is desired to coat both surfaces and air knife doctor both coated surfaces simultaneously. The arrangement of the coating apparatus may be similar to that disclosed in God, US. 2,423,768 except that in place of curved portions attached to the leading or lower lip of two air knife doctors located on opposing positions on opposite sides of the web, taut wires are placed in position in the same relationship with the air knife doctor and the web as described above.

While the examples have been directed to coating impervious films with photographic emulsions and auxiliary layers, the invention is by no means limited thereto. Other web materials such as paper and cloth may also be advantageously coated with the novel coating arrangement. Other liquid coating solutions may also be applied by the novel apparatus. For example, gelatin and resin sizing solutions used in the treatment of paper and cloth may be applied in this manner. In the photographic art, such layers as separator, filter, and antiabrasion layers may also be applied in this manner. Although the doctor is referred to as an air knife doctor, other gaseous fluids such as steam or insert gases e.g., nitrogen, helium, etc., may be used depending on the nature of the coating. The outstanding advantage of the invention is that extremely high coating speeds, e.g., up to 400 feet per minute may be employed without the usual coating defects of streaks, unevenness, etc.

What is claimed is:

1. In an apparatus for applying a smooth coating of desired wet thickness of coating material onto a moving web including, means for applying to said web a layer of fiowable coating material of at least 200% of said desired wet thickness, an air knife doctor means for directing a stream of air at said layer at an impingement line transverse to the direction of web travel, the improvement which consists in having a taut Wire or rod positioned parallel to said impingement line at a distance p from the coated Web, at a distance q below the impingement line and said wire or rod having an effective diameter d wherein:

d is a distance equal to from 4 to 20 times the said desired wet thickness,

p is a distance equal to from about 1 to 6 times the said desired wet thickness, and

q is a distance equal to from 1 to 5 times d.

2. Apparatus according to claim 1 wherein distance q is from about 0.06 inch to about 0.25 inch, and diameter a is from about 0.02 inch to about 0.1 inch.

3. In an apparatus for applying a smooth coating of desired wet thickness of coating material onto a moving web including, means for applying to said web a layer of fiowable coating material of at least 200% of said desired wet thickness, an air knife doctor means for directing a stream of air at said layer at an impingement line transverse to the direction of Web travel, the improvement which comprises having a thin, rounded longitudinal obstruction positioned parallel to said impingement line at a distance p from the coated web and at a distance q below the impingement line, said rounded longitudinal obstruction having an effective diameter d wherein:

at is a distance equal to from 4 to 20 times the said desired Wet thickness,

p is a distance equal to from about 1 to 6 times the said desired Wet thickness, and

q is a distance equal to from 1 to 5 times d.

4. Apparatus according to claim 3 wherein at least one additional thin, rounded longitudinal obstructionds positioned below said rounded longitudinal obstruction equidistant from said coated web.

References Cited in the file of this patent UNITED STATES PATENTS 2,386,156 Woodward Oct. 2, 1945 2,679,231 Pamper et a1 May 25, 1954 

